system appendPDF cover-forpdf · Draft The right choice: predation pressure drives shell selection decisions in hermit crabs Journal: Canadian Journal of Zoology Manuscript ID cjz-2017-0023.R2

Draft

The right choice: predation pressure drives shell selection

decisions in hermit crabs

Journal: Canadian Journal of Zoology

Manuscript ID cjz-2017-0023.R2

Manuscript Type: Article

Date Submitted by the Author: 10-Oct-2017

Complete List of Authors: Arce, Elsah; Universidad Autonoma del Estado de Morelos, ; Universidad Nacional Autonoma de Mexico, Cordoba, Alejandro; Universidad Nacional Autonoma de Mexico, Departamento de Ecología Evolutiva, Instituto de Ecología

Keyword: Calcinus californiensis, hermit crab, gastropod shell, mistake, Shelter choice

https://mc06.manuscriptcentral.com/cjz-pubs

Canadian Journal of Zoology

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The right choice: predation pressure drives shell selection decisions in hermit crabs 1

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E. Arce1,2*

and A. Córdoba-Aguilar2*

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1Laboratorio de Acuicultura, Departamento de Hidrobiología, Centro de Investigaciones Biológicas, 5

Universidad Autónoma del Estado de Morelos, Cuernavaca, Morelos, México. Tel: +52-77-73162354. E-6

mail: [email protected] 7

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2Departamento de Ecología Evolutiva, Instituto de Ecología, Universidad Nacional Autónoma de México, 9

Apdo. P. 70-275, Ciudad Universitaria, 04510, México, D F., México. Tel: +52-55-56229003. E-mail: 10

[email protected] 11

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*Corresponding authors: 13

[email protected] & [email protected] 14

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The right choice: predation pressure drives shell selection decisions in hermit crabs 16

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E. Arce and A. Córdoba-Aguilar 18

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Abstract 20

Several prey species use refuges to avoid predation. Prey need to abandon and shift between refuges. 21

However, during such shifting, prey can be vulnerable to predators. We hypothesize that predator presence 22

may induce prey to make mistakes in choosing their refuge. We tested this by using hermit crabs Calcinus 23

californiensis Bouvier, 1898 that were induced to shift to a new empty gastropod shell (three different 24

species: Columbella sp. Lamarck, 1799, Nerita scabricosta Lamarck, 1822, and Stramonita biserialis 25

(Blainville, 1832)) in the absence and presence of a natural crab predator, Eriphia squamata Stimpson, 1860, 26

an efficient shell-crushing predator. We expected that when a predator was present, hermit crabs would a) 27

inspect fewer shells and/or b) change to a shell that is either too heavy to allow escape or unfit in size to 28

accommodate the hermit crab. While the first prediction was met, the second prediction was supported only 29

when S. biserialis shells were used. Thus, in the presence of a predator, hermit crabs prioritize escaping via 30

selecting lighter shells, which would allow the crab to move faster. We conclude that predator presence may 31

induce prey to make mistakes in refuge selection, suggesting that this has severe consequences in future 32

predatory events. 33

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Keywords: shelter choice, mistake, gastropod shell, Calcinus californiensis, hermit crab 35

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Introduction 38

Predators are a major selection pressure on prey, thus there has been selection for general adaptations to avoid 39

detection, capture and ingestion in many prey species. Some examples of prey adaptations are alterations in 40

activity, avoidance of sites occupied by predators, and an increase in their use of refuges (Brönmark and 41

Miner 1992; Sih et al. 1992; Skelly 1994). It is well known that a refuge increases prey survival rate (i.e. 42

Bertness 1981; Shervette et al. 2004). However, prey animals must find new refuges occasionally to avoid 43

fitness costs in terms of growth (Lankford et al. 2001; Heithaus and Dill 2002) and reproduction (Spencer 44

2002; Auld and Houser 2015). This shifting between refuges exposes the animal to the cost of becoming 45

exposed to predators at least temporarily. When facing such a situation, prey may make mistakes in choosing 46

a new refuge, as prey must direct their attention to not only inspecting potential refuges but also to avoid 47

capture by the predator. This means, for example, a reduced time to inspect potential refuges with the 48

remaining time devoted to detecting and avoiding the predator. 49

Hermit crabs are good study subjects to test the effect of predator threat on decision-making during 50

refuge shifting (Hazlett 1971). These animals are characterized by a soft abdomen that requires protection 51

from environmental stress and predation (Resse 1969; Vance 1972). Shell use is dependent on shell 52

availability (Turra and Leite 2002; Arce and Alcaraz 2011) and preference (Meireles et al. 2008; Arce and 53

Alcaraz 2012). The size and type of selected shells have direct effects on crab fitness. The proper size and 54

type of shell may allow greater clutch sizes (Childress 1972; Elwood et al. 1995), higher growth rates 55

(Asakura 1992; Alcaraz et al. 2015), lower risk of predation (Angel 2000; Arce and Alcaraz 2013) and less 56

environmental stress (Bulinski 2007; Argüelles et al. 2009). It is also related to its morphological structure 57

and dimensions; for example, tight shells offer more protection from being pulled out by predators (Arce and 58

Alcaraz 2013), while thick shells are effective against shell-crusher predators (Bertness and Cunningham 59

1981). Nevertheless, hermit crabs frequently need to move into a new shell. During shell shifting, hermit 60

crabs have to search, inspect, handle and change shells. These different steps can take time during the process 61

of discarding and trying different shell sizes. It is during this move that hermit crabs run the risk of being 62

predated on. Thus, the influence of a predator threat may induce hermit crabs to make mistakes in shell 63

selection given the reduced time because of avoiding predation. To our knowledge, the effect of predation 64

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threat on the chance of making mistakes in shell selection has not been examined in hermit crabs or other 65

animals. Although it is well known that predation risk can ultimately affect hermit crab preference and shell 66

change (Mima et al. 2003; Briffa et al. 2008; Arce and Alcaraz 2013), these consequences can occur after a 67

crab makes the mistakes. Thus, we have herein examined shell selection in the face of predation risk in hermit 68

crabs and during shell shifting. For this, we used Calcinus californiensis Bouvier, 1898 hermit crabs and 69

Eriphia squamata Stimpson, 1860 crab predators. The latter is a natural predator of C. californiensis that 70

crushes the shell to reach the hermit crab (Bertness and Cunningham 1981). According to this predation 71

strategy, hermit crabs should select light shells so that they can escape more effectively (Alcaraz and Arce 72

2017). Assuming that such crushing predators induce prey to make more mistakes, we therefore predicted that 73

during the presence of a predator, crabs would a) inspect and/or change fewer shells, and b) select a shell that 74

is not appropriate in terms of weight and/or size. 75

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Materials and methods 77

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Collection and maintenance 79

The animals used did not involve endangered or protected species. All applicable international, national 80

and/or institutional guidelines for the care and use of animals were followed. Hermit crabs (C. californiensis) 81

and predators (E. squamata) used for experimental trials were collected by hand in an intertidal zone at 82

Troncones, Guerrero, Mexico (17°47’16’’N; 101°44’17’’W) in November and December 2014. Crabs were 83

transported to the laboratory and maintained in individual containers to avoid shell exchange. Animals were 84

removed from their shells and measured (shield length in mm and hermit crab mass in mg). We only used 85

males of both hermit crabs and predators in our trials. For both, only animals that had all their appendages and 86

shells with no clear damage or epibionts were used. All animals were kept under a natural light:dark 87

photoperiod with running seawater (35 PSU) at an approximate temperature of 28°C. Crabs were fed with 88

sinking pellets (®New Spectrum) once a day. Predators were maintained in individual containers and fed with 89

hermit crabs 24 h prior to experiments. In all tests described below, different hermit crabs were used. On 90

completion of trials, all animals were returned to the sea. 91

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Shell selection during predator absence and presence 93

A hermit crab was placed with its original shell, when this was one of the most commonly occupied shells by 94

C. californiensis (Arce and Alcaraz 2011, 2012). For instance, the one it was occupying when captured in the 95

field, either Columbella sp. Lamarck, 1799, Nerita scabricosta Lamarck, 1822 or Stramonita biserialis 96

(Blainville 1832) and with 20 other empty shells of the same species the crab was using in a 20-L individual 97

glass tank for 24 h. The 20 shells offered varied in size. Each shell was previously weighed (Shell Mass, 98

ShM), measured (using four morphometric variables: shell length, ShL; shell width, ShW; shell aperture 99

length, ShAL and shell aperture width, ShAW) and marked with different color combinations. Direct 100

observations and measurements were recorded every 6 hours. In this time, we recorded two behavioral aspects 101

of the hermit crabs under these conditions (1 – the number of shell inspections). A shell inspection took place 102

when the hermit crab held a shell with appendices for review, but discarded it (2 – the number of shell 103

changes). A shell change took place when the hermit crab abandoned its own shell and moved into a new 104

shell. We assumed that a hermit crab preferred a shell when it occupied it for as long as 24 h. 105

We then carried out a similar experiment in which E. squamata predators were introduced (Zipser 106

and Vermeij 1978). These predators were fed with hermit crabs 24 h before the experiment started and were 107

then placed in a 40-L glass tank. This tank was divided into two halves (20 L each) using a rigid and 108

transparent mesh (5 mm). Water was allowed to circulate through the tank so that the chemical cues from E. 109

squamata reached the hermit crab. We again recorded the number of shell inspections, shell changes and shell 110

selection for 24 h. 111

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Shell preference and the right choice 113

Shell preference in crabs occupying Columbella sp, N. scabricosta, and S. biserialis was assessed using the 114

following combinations: hermit crabs in Columbella sp. in the absence of a predator (n = 15) and in the 115

presence of a predator (n = 17); hermit crabs in N. scabricosta in the absence of a predator (n = 16) and in the 116

presence of a predator (n = 14); and hermit crabs in S. biserialis in the absence of a predator (n = 15) and in 117

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the presence of a predator (n = 16). The shell selected by the crab – after it remained in it for a 24-hour period 118

– was considered preferred. The number of “mistakes” was recorded during the shell selection process. A 119

mistake was made when a hermit crab was occupying a shell out of its preferred weight and/or size (more 120

than 10% tight or more than 10% loose; see Arce and Alcaraz 2012). The result of the preference for shells 121

according to predator absence/presence was compared to determine whether the presence of a predator affects 122

the decision. 123

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Statistical analyses 125

The numbers of shell inspections and shell changes according to predator absence/presence were compared 126

using a chi-square test (χ2). Relationships between crabs and gastropod shells sizes were established by 127

regression analyses, where x = hermit crab size (M) and y = shell size (ShM or ShAW; Arce and Alcaraz 128

2012). The preferred shell was determined using this regression, which was performed for each shell species. 129

The slopes and elevations were compared using ANCOVA analysis of covariance to estimate the differences 130

between the shell size selected under predator absence and under predator presence. We also used ANOVA to 131

compare the number of mistakes in predator absence versus predator presence. All reported values are means 132

± SE. Statistical analyses were conducted using Statistics 7®. 133

134

Results 135

Hermit crabs inspected fewer gastropod shells when a predator was present than when it was absent (ᵡ2(2,1) = 136

28.46, p < 0.001; Fig. 1). This situation prevailed independently of the gastropod species (F(5,56) = 15.91, p < 137

0.001; Fig. 1). 138

Hermit crabs showed more shell changes during predator absence than during predator presence 139

(ᵡ2(2,1) = 8.91, p < 0.01; Fig. 2) and when hermit crabs occupied Columbella sp. shells during predator absence, 140

compared to occupation of S. biserialis and N. scabricosta (F(5,56) = 18.50, p < 0.001; Fig. 2). Finally, when 141

the predator was present, shell changes did not differ among the three shell species (Fig. 2). 142

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Hermit crabs selected different shell sizes or weights when a predator was absent than when the 143

predator was present (Fig. 3). Specifically, hermit crabs occupying Columbella sp. (F(2,30) = 15.62 p < 0.001, 144

Fig. 3a), N. scabricosta (F(2,28) = 18.86 p < 0.001, Fig. 3b) and S. biserialis (F(2,29) = 16.78 p < 0.001, Fig. 3c) 145

preferred smaller or lighter gastropod shells when a predator was present. 146

The number of mistakes during the shell selection process was different according to the predator 147

absence/presence condition and shell species occupancy. When hermit crabs occupied Columbella sp. and N. 148

scabricosta shells – in the absence of a predator – they made more mistakes than when the predator was 149

present (Table 1). However, when hermit crabs occupied S. biserialis shells and the predator was absent, they 150

made fewer mistakes than when the predator was present (Table 1). 151

152

Discussion 153

We postulated that although the presence of a predator may elicit behavioral responses to avoid predation in 154

prey, this pressure might also affect prey decisions. In concordance with our first prediction, hermit crabs 155

made fewer changes when a natural predator was present. Moreover, our results supported our second 156

prediction, since crabs made more mistakes when a natural predator was present in terms of choosing a 157

suboptimal shell but only when using S. biserialis shells. Previous studies also found that animals modify 158

their behavior in the presence of predators; for example, hermit crabs modified their shell selection behavior 159

in the presence of a predator (Rotjan et al. 2004; Arce and Alcaraz 2013; Alcaraz and Arce 2017) and other 160

crustaceans showed less activity when predation risk was high (Hazlett 1996; Percor and Hazlett 2003). These 161

results, in addition to the results of the present study, imply that hermit crabs can derive less information 162

about resource value, in this case, the utility of the shell as an effective refuge, in the face of predator 163

pressure. Related to predator presence, one antipredator response that hermit crabs show is a retraction inside 164

the occupied shell (Briffa and Elwood 2001), which is a generalized response in animals using refuges (Sih et 165

al. 2004). However, one immediate consequence of retracting is that hidden animals lose feeding and 166

reproductive opportunities (Reaney 2007). For example, when the predator Carcinus maenas (Linnaeus 1758) 167

is present, the prey Nucella lapillus (Linnaeus 1758) reduces foraging activity, as it remains hidden for longer 168

(Vadas et al. 1994). The same result can occur even in prey that does not rely on refuge (Matassa and Trussell 169

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2014). This is the case, for example, of lepidopteran larvae that reduce their foraging rate when the perceived 170

level of predation risk is high (Berger and Gotthard 2008). 171

Prey-catching strategies by predators play key roles in how well prey can determine an effective 172

refuge size (Walters and Wethey 1996). The risk of predation in hermit crabs is strongly related to shell size 173

relative to crab size. According to previous studies (i.e. Arce and Alcaraz 2013), we used this relationship as a 174

tool to determine whether crabs had made the right choice of refuge size, because the fit is essential for hermit 175

crabs during predation attacks. In relation to this, C. californiensis deals with two types of natural predators in 176

their habitat: peelers and crushers. Peelers (e.g. the crab Arenaeus mexicanus (Gerstaecker 1856)) pull out the 177

crab by directly cutting and removing parts of the crab, while crushers (i.e. the predator we used, E. 178

squamata) break the shell with their chelae (Bertness and Cunningham 1981). Interestingly, each predator 179

style performs better with certain species of shell than others. For example, some shells allow the hermit crab 180

to retreat completely and become unreachable to a peeler predator (Angel 2000). However, such shells are 181

ineffective if the predator is a crusher. With a crusher, the hermit crab has to flee before its shell is grabbed. In 182

this latter circumstance, a hermit crab does better if it bears a light shell, as it allows faster movements. 183

According to this context, we found that C. californiensis crabs selected tighter or lighter shells when they 184

perceived a threat by E. squamata. This is concordant with results from previous studies that suggest light 185

shells allow hermit crabs to move faster (Herreid and Full 1986; Alcaraz and Arce 2017) and escape 186

effectively when faced with a predator. Thus, the use of light shells by C. californiensis may be adaptive to 187

avoid attacks by the crusher E. squamata (Alcaraz and Arce 2017). However, when choosing light shells, the 188

crab made more mistakes if it faced a predator when occupying S. biserialis (the preferred shell; Arce and 189

Alcaraz 2012). This shell species may have some additional yet undetected costs, which may only arise when 190

a predator is present. Future studies can be conducted to clarify this. 191

Individuals may benefit from modifying their resource searching behavior when they perceive 192

predation risk. However, this perception may induce them to make mistakes during shell selection. An 193

incorrect choice and/or suboptimal shelter does not provide the individual with the benefits of counteracting a 194

predatory attack. In this context, our study demonstrated that resource selection in the face of predation might 195

induce mistakes by restricting inspection to fewer shells, which may have severe consequences for future 196

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predatory events. To our knowledge, we have provided the first evidence of the occurrence of such mistakes 197

and the costs prey incur from these mistakes when a predator is present. 198

199

Acknowledgements 200

We thank the DGAPA-UNAM Postdoctoral Program for providing a full scholarship to the first author. We 201

thank Karla Kruesi for technical support in the field and the laboratory, Suntai Sotola and Manuel De La 202

Torre for the drawings for Figures, and Guillermina Alcaraz for logistic and academic support. 203

204

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Figure captions: 299

300

Figure 1. Number of shell inspections by hermit crabs occupying different gastropod shell species 301

during predator absence/presence scenarios. Different letters indicate significant differences between 302

treatments, while different numbers indicate differences between gastropod shell species (p < 0.001). 303

304

Figure 2. Number of shell changes by hermit crabs occupying different gastropod shell species 305

according to predator absence/presence scenarios. Different letters indicate significant differences 306

between treatments, while different numbers indicate differences between gastropod shell species (p 307

< 0.001). 308

309

Figure 3. Regression slopes according to shell size preference by hermit crabs: A) Columbella sp., 310

B) Nerita scabricosta and C) Stramonita biserialis. R2, correlation coefficient (p < 0.001). 311

312

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314

315

316

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318

319

320

321

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Table 1. Analysis of variance testing the number of mistakes hermit crabs made when getting 323

into a new shell species under predator absence/presence scenarios. 324

325

Gastropod shell Predator

stimulus

Mistakes

(n)

SE df F P

Columbella sp.

Absence 54 0.94

1 10.33 <0.01

Presence 25 0.89

N. scabricosta

Absence 52 0.64

1 7.35 <0.01

Presence 31 0.89

S. biserialis

Absence 37 0.47

1 64.69 <0.001

Presence 66 0.20

326

327

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Figure 1 328

329

330

331

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Figure 2 332

333

334

335

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Figure 3 336

y = 4.4101x + 2.0394R² = 0.731

y = 2.8644x + 1.4922R² = 0.3676

0

1

2

3

4

5

0 0.05 0.1 0.15 0.2

Shell Aperture Length (mm)

Hermit crab mass (g)

Predator presence

Predator absenceA

y = 1.9333x + 0.337R² = 0.7767

y = 1.6898x + 0.0946R² = 0.721

0

0.5

1

1.5

0 0.1 0.2 0.3 0.4

Shell mass (g)


Predator presence

Predator absenceB

y = 4.1657x + 0.9666R² = 0.7928

y = 3.6477x + 0.4541R² = 0.8671

0

1

2

3

4

5

0 0.2 0.4 0.6 0.8 1

Shell mass (g)


C

Predator presence

Predator absence

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Documents

system appendPDF cover-forpdf · Draft The right choice: predation pressure drives shell selection decisions in hermit crabs Journal: Canadian Journal of Zoology Manuscript ID cjz-2017-0023.R2